Method of drying wood

ABSTRACT

A method of rapidly drying wood by combining radio frequency or microwave heating with circulated heated air kiln drying, in which the surface temperature of the wood is measured, the wet and dry bulb temperatures of the circulated heated air within the kiln are measured, the wet bulb temperature is maintained according to a kiln schedule for the species and thickness of the wood, and in addition the input of radio frequency or microwave energy and the dry bulb temperature of the kiln are regulated to control the surface temperature of the wood according to the dry bulb temperature of the kiln schedule.

[ 51March '20, 1973 [54] METHOD OF DRYING WOOD Donald G. Miller, Ottawa, Ontario, Canada [75] Inventor:

[73] Assignee: Canadian Patens and Development Limited, Ottawa, Canada [22] Filed: June 4, 1971 [21] Appl. No.: 149,912

10/1969 Byron ..73/355R 2/1970 Zottu .3411

Primary ExaminerCarroll B. Dority, Jr. Attorney-James R. Hughes [5 7 ABSTRACT A method of rapidly drying wood by combining radio frequency or microwave heating with circulated heated air kiln drying, in which the surface tempera-I ture of the wood is measured, the wet and dry bulb temperatures of the circulated heated air within the kiln are measured, the wet bulb temperature is maintained according to a kiln schedule for the species and thickness of the wood, and in addition the input of radio frequency or microwave energy and the dry bulb temperature of the kiln are regulated to control the surface temperature of the wood according to the dry bulb temperature of the kiln schedule. I

4 Claims, 4 Drawing Figures PATENTEDMAR201975 SHE-ET 30F 3 mmDOI l 22:. wz mQ Om METHOD OF DRYING WOOD This invention relates to a rapid method of drying wood.

Wood is dried by the evaporation of surface moisture. When this occurs, moisture from the zone immediately below the surface moves to the drier surface zone and thus a moisture flow is set up from the interior to the surface. A controlling factor is the rate at which the surface moisture is picked up by the surrounding air. This is influenced by the temperature and circulation of the surrounding air. If moisture could be moved from the interior to the surface as rapidly as it can be evaporated from the surface, wood seasoning would be a simple process. Unfortunately this is not the case and the rate of surface evaporation must be carefully regulated according to the rate of transfer of moisture from the interior. If the surface dries below the fiber saturation point too quickly in relation to the drying of the core, the surface attempts to shrink but is restrained by the core which has not shrunk because its moisture content is still above the fiber saturation point. This shrinkage force on the surface may be greater than the strength of the wood in tension perpendicular-to-the-grain with the result that surface checks develop to relieve the stress. Alternatively, this tension stress may cause the surface to set in an expanded condition. As drying continues, the core dries out and attempts to shrink but is held in an expanded condition by the permanent set" of the surface. This condition can result in internal checks (honeycombing), case-hardening and collapse.

In conventional kiln drying of wood, the rate of surface evaporation is regulated according to a drying schedule which specifies the dry and wet bulb temperatures of the air circulating in the kiln for the species and thickness of the wood being dried and for the stage of the drying process. These schedules are based on the results of years of kiln drying research and experience. The following schedule for 2-inch spruce, taken from Millet R.S., Kiln-drying of lumber in eastern Canada," Technical Note No. 21, Ottawa Laboratory, Department of Forestry, Ottawa, 1961, is typical.

Equilibrium when the Temperature F Relative moisture percentage moisture Dry Wet Wet bulb humidity content content of wood is: bulb bulb depression (percent y over 40, use 140 133 7 81 14.3 40-30, use 150 135 66 9.8 30 25, use 160 130 30 43 5.3 25, use 180 130 50 26 3.0 Conditioning 180 171 9 81 12.6

Using this schedule at least 60 hours are required to dry 2-inch spruce from 60 percent moisture content to 15 percent moisture content. The drying of 2-inch birch by the appropriate kiln schedule requires several weeks. There is obviously a need for a more rapid method of drying wood.

Ever since radio frequency generators became commercially available about 30 years ago, investigators all over the world have been intrigued with the idea of using radio frequency energy to rapidly dry wood. Compared to more conventional methods of heating, the unusual feature of radio frequency heating is that the heat is generated within the wood itself. Surface cooling can cause the interior of the wood to be at a higher temperature than the surfaces, suggesting that the heating is proceeding from the inside out. Radio frequency heating appears to be an ideal method of drying wood: it can heat the wood quickly and there is a temperature gradient to increase the rate of moisture movement from the interior to the surface.

Early investigators were encouraged by the fact that small, short pieces of pervious wood can be dried very quickly and without degrade using a high concentration of radio frequency power. The temperature of the wood is quickly brought to the boiling point and the water is forced out due to internal steam pressure. When this boiling method is used to dry large pieces of wood moisture cannot escape fast enough to relieve the internal steam pressure and the lumber is usually ruptured by steam explosions.

In 1948, the inventor published a paper entitled Application of dielectric heating to the seasoning of wood, 1948, Proc. F.P.R.S. pp. 235 to 241, describing the temperature gradient method of radio frequency drying. In this method the magnitude of radio frequency power is adjusted to maintain the interior of the wood at a temperature safely below the boiling point of water. The wood and radio frequency electrodes are placed inside a kiln in which the air is circulated and kept very humid and relatively cool. A temperature gradient is established which causes moisture to move from the center of the wood to the surface. One disadvantage of the temperature gradient method is that much of the heat produced in the wood by the relatively expensive radio frequency energy is deliberately taken away by the circulating air to establish a temperature gradient. For this reason, the cost of drying by the temperature gradient method is undesirably high.

A further disadvantage of the temperature gradient method is that excessive surface drying in the early stages of the drying causes the wood to be damaged by external or internal checks. Although the surface of the wood is at a lower temperature than the interior wood, it is still at a higher temperature than the air circulating in the kiln. Therefore the relative humidity close to the wood surface is lower than that of the circulating air and this causes excessive surface drying.

Wood (U.S. Pat. No., 2,567,983 dated Sept. 18, 1951) combined radio frequency heating with circulating hot air but he failed to see the need for measuring the surface temperature of wood and of controlling this temperature with respect to the wet and dry bulb temperatures of the circulating air so as to prevent defects due to excessive surface drying and so as to prevent waste of heat produced by radio frequency energy. For this reason Woods drying process is costly and is limited to the final drying of partially dried lumber.

In the drying process developed in England and described in U.S. Pat. No. 3,083,470 dated Apr. 1963, J .H. Pless, the surrounding atmosphere is maintained at a temperature of about 200 F and made humid by injecting steam. These conditions help to prevent exces- 'sive surface drying of green wood but the high humidity retards the drying after the moisture content is below the fiber saturation point. In Pless process however, there is no forced air circulation and the relative humidity of the surrounding atmosphere is not measured.

A more recent development is the drying process developed by Zottu (U.S. Pat. No. 3,496,645 dated Feb. 24, 1970) in which radio frequency heating is combined with an atmosphere which is always at a temperature of 212 F or higher and which always has a high humidity. In the vernacular of the lumber industry, Zottus process combines radio frequency heating with high temperature kiln drying. In high temperature kiln drying, a schedule such as that shown on page 2, lines 6-14 of this application, is not used. Instead, the drying is controlled by regulating only the temperature of the saturated atmosphere according to an established relationship (Principles of Wood Science and Technology, I, Solid Wood, published by Springer Verlag New York Inc. 1968, Kollman and Cote Jr., page 461 In Zottus method, the surface temperature of the wood is not measured.

It is an object of my invention to provide a rapid method of drying wood without the occurrence of sur face checks, internal checks (honeycombing), casehardening and collapse in the dried wood.

According to the invention there is provided a method of drying wood comprising combining dielectric heating of the wood with circulated heated air kiln drying of the wood, measuring the surface temperature of the wood, measuring the dry and wet bulb temperatures of the circulating heated air within the kiln, maintaining the wet bulb temperature of the circulating heated air substantially at a predetermined value, regulating the dry bulb temperature of the circulating heated air and regulating the input of electrical energy for said dielectric heating so as to maintain the surface temperature of the wood substantially at a predetermined value, said predetermined values being based on a conventional schedule appropriate for the wood species, thickness and stage of drying, and said dielectric heating being selected from the group radio frequency heating and microwave heating.

In the accompanying drawings which illustrate, by way of example, an embodiment of the invention,

FIG. 1 is a partly sectional side view of an apparatus for drying wood,

FIG. 2 is an enlarged isometric side view of a portion of an electrode assembly in FIG. 1,

FIG. 3 is an end view of FIG. 2 in the direction of arrow III, and

FIG. 4 is a graph of drying curves for white spruce.

Referring to FIG. 1 there is shown a kiln 1, a circulating fan 2, a kiln truck 3, a radio frequency generator 4, a dry bulb temperature sensor 6, a wet bulb temperature sensor 8, an automatic controller and recorder of dry bulb and wet bulb temperatures within the kiln 1, a calcium fluoride glass window 12, an infrared radiation thermometer 14 for operating at a wavelength of 3.4 microns, and a controller and recorder 16.

The kiln 1 has an adjustable vent 18, steam heating pipes 19, and a steam inlet jet nozzle 20. These are controlled by the controller portion of controller and recorder 10. A false ceiling 22 is provided in the kiln l and is mounted above rails 24 and 25 leading into the kiln 1. The fan 2 comprises fan blades 26 mounted on a shaft 28. The shaft 28 is rotatably mounted in brackets 29 and 30 secured on the ceiling 22. The shaft 28 extends through a wall 32 of kiln, within which it is rotatably sealed, and has a pulley 34 keyed on its end. An electric motor 36 has a pulley 38 keyed on to its driving shaft, and a belt drive 40 (shown chain-dotted) connects the pulleys 32 and 40.

The kiln truck 3 is movable along the rails 24 and 25 by four wheels, two of which are shown and designated 42 and 44. A plurality of layers 46 of rows of timbers are mounted upon the kiln truck 3 and are separated by electrode assemblies 48, 48a, 48b and 480 in a manner which will be described later with reference to FIGS. 2 and 3.

The radio frequency generator 4 has a high voltage output terminal 50 connected by a cable 52 to the central electrode 48a at a central position in the height of the layers 46, which is a high voltage electrode. The top and bottom electrodes, 48b and 48c respectively, are connected by a ground cable 54 and are ground electrodes. The electrode 48c is connected by a ground cable 56 to the radio frequency generator 4. The cables 52 and 56 pass through an insulator 58 in the wall of the kiln 1.

Referring to FIGS. 2 and 3 the electrode assemblies 48, 48a, 48b and 480 each comprise an aluminum sheet 60 which is perforated with 3/l7-inch diameter perforations or holes, some of which are shown and designated 62, on A. inch staggered centers, that is the centers of adjacent holes are at the ends of two lines /4 inches in length and forming a right angle. The sheet 60 extends the length of the timber layers 46 (FIG. 1) and has two 15 inch by A inch square cross-section aluminum bars 64 and 66 extending along both sides. The bars 64 and 66 have slots 68 and 70 respectively, into which spacers 72 and 74 extend and are clamped by screws 76 and 77. The spacers 72 and 74 extend across the whole width of the sheet 60 and are preferably of a material such as polystryrene having a relatively low radio frequency loss factor, however, spacers 72 and 74 of maple wood have been found to be suitable. The spacers 72 and 74 in this embodiment are spaced at 12 inch intervals along the sheet 60, and screws 77 are disposed midway between them.

In operation the apparatus is arranged as shown in FIG. 1 with the timber layers 46 extending lengthwise along the electrode assemblies 48, 48a, 48b and 48c, and the timbers in each layer spaced apart to permit air to circulate around them.

The motor 36 is started to drive the fan 2 to circulate air between the timber layers 46 and through the perforations or holes 62 (FIG. 2), steam is passed along the steam heating pipes 19, and steam is introduced into the kiln 1 by the nozzle 20.

The kiln drying of the timber layers 46 then proceeds according to a kiln schedule for the particular species and thickness of wood forming the timber layers 46. For example, in the case of green spruce 2 inches in thickness, the kiln controls are set to maintain the dry bulb temperature of the air within the kiln l at F, the wet bulb temperature at 133 F, and air being circulated by the fan 2 at about 600 feet per minute.

When these conditions have been established the radio frequency of 2 MHz or other suitable frequency from the radio frequency generator 4 passes from the electrodes assembly 48 through each timber layer 46 and each electrode assembly, in an upwardly direction to electrode assembly 48b, and in a downwardly direction towards electrode assembly 480. As previously described, the radio frequency energy passing through the timber generates heat within it and drives moisture from the center of the timber to the outer surface.

The dry bulb temperature and the wet bulb temperature of the atmosphere surrounding the timber layers 46 are recorded on the recorder portion of 10, and the surface temperature of the wood is measured by means of the infrared radiation thermometer 14 and is recorded on the recorder portion of the controller and recorder 16.

After about 1 hour, the surface temperature of the timber layers 46 starts to climb above 133 F, and when it reaches 140 F control of the dry bulb temperature is manually transferred to the infrared thermometer 14 which regulates the dry bulb temperature so as to limit the surface temperature of the timber layers 46 to 140 F. The drying is continued in this manner and the dry bulb temperature is automatically lowered as required to limit the surface temperature of the timber layers 40 to about 140 F. The drying is continued in this fashion until the moisture content of the timber layers 46 is below 40 percent by weight. Then in accordance with the schedule the wet bulb temperature is raised to about 135 F and the dry bulb temperature is permitted to rise until the surface temperature of the timber layers 46 climbs to about 150 F. Again the controller portion of regulates the dry bulb temperature to limit the surface temperature to about 150 F. The drying continues and the kiln 1 is adjusted as indicated above in accordance with the kiln schedule.

At the start of this drying process when the surface temperature is lower than the dry bulb temperature of the circulating air, a relatively high air circulation (600 feet per minute or greater) is preferable because the flow of heat is from the air to the wood. Later in the drying when the surface temperature of the wood is higher than that of the circulating air, the air circulation should preferably be reduced to about 100 feet per minute or less in order to limit the flow of heat from the wood to the circulating air.

Novel features of the combination of radio frequency heating and kiln drying according to the invention are, when taken together:

1. The surface temperature of the wood being dried is continuously measured.

2. The wet bulb temperature of the kiln air is maintained at the value called for in the appropriate kiln schedule.

3. The input of radio frequency power and the dry bulb temperature of the kiln air are regulated to maintain a difference between the surface temperature of the wood being dried and the wet bulb temperature substantially equal to the difference between the dry bulb temperature and the wet bulb temperature called for in the schedule.

4. The input of radio frequency power and the dry bulb temperature of the kiln air are regulated to maintain a controlled difference between the surface temperature of the wood being dried and the dry bulb temperature.

These features when taken together are important because they bring the drying process under control, and thereby make it both technically and economically feasible. By maintaining the wet bulb temperature called for in the kiln drying schedule and by maintaining the appropriate relationship between the surface temperature of the wood being dried and the wet bulb temperature, the rate of surface evaporation is controlled so that seasoning defects such as surface and internal checking are avoided. At the same time, the higher internal temperature of the wood being dried in the process according to the invention, than that of known processes, provides a temperature gradient from the interior of the wood to its surface which accelerates the movement of moisture from the wood interior to its surface, thereby greatly reducing the drying time to that of known processes. By regulating the radio frequency input power and the dry bulb temperature to maintain a controlled difference between the surface temperature of the wood being dried and the dry bulb temperature, the flow of heat from the wood to the circulating kiln air is controlled to prevent excessive loss of this heat which in this embodiment is produced from expensive radio frequency energy. This factor makes the process according to the invention more economically feasible than other processes using radio frequency heating.

Using the above-described method, 2-inch white spruce was dried from about 60 percent moisture content by weight to about 15 percent moisture content by weight within approximately 15 hours. The quality of the seasoning was excellent. There were no exterior or interior checks and the moisture content was substantially uniform throughout the cross-section of the timber. The time required to dry this timber by conventional kiln drying alone would be about 60 to hours.

FIG. 4 shows typical graphs of the drying time in hours versus the percentage of moisture content by weight for undressed white spruce 2 inches thick. Graph is for the white spruce dried by a conventional kiln process and graph 82 is for matched white spruce dried by the process according to the invention.

It will be noted that the slope of graph 80 gradually decreases as the drying progresses. On the other hand, the slope of graph 82 is essentially constant, indicating a nearly constant rate of drying even at the lowest moisture content.

An important feature of the method according to the invention is that it does not require special new drying schedules to be developed. Wood of any species and moisture content can be dried by the process according to the invention without experimenting provided that there is available an existing good drying schedule for that species.

It will be appreciated that the process according to the invention if of interest to lumber producers having existing wood drying kilns as well as those installing new kilns. Radio frequency heating could be added to these existing kilns to speed up the drying and thereby increase the output of the existing kiln. The gain in output would be achieved without the need for much additional floor space or steam requirements.

It is within the scope of the invention to use microwave energy as the dielectric heating for heating the wood to be dried instead of radio frequency energy. However, the present costs of microwave heating make radio frequency heating the most attractive form of heating.

The overall efficiency of a radio frequency generator in converting 60 cycle power into radio frequency energy is about 50 percent. This means that approximately one half of the 60 cycle power is converted into heat which is normally wasted. Because this heat is normally at a lower temperature than the required kiln temperatures, in some instances it is not practical to use this energy directly in the process of the invention. However, this heat from the radio frequency generator could be utilized to good advantage for preliminary low temperature drying of green timber and for heating dry storage areas.

In a different embodiment, the spacer 72 (FIG. 2) is shaped so that the portion between the bars 64 lies flush with the upper surface of the bars 64, the spacer being similarly shaped to lie flush with the bars 66.

In another embodiment, a plurality of electrodes 48, 48a, 48b and 480 are disposed side-by-side across the plurality of layers 46 for ease of handling.

I claim:

1. A method of drying wood comprising combining dielectric heating of the wood with circulated heated air kiln drying of the wood, measuring the surface temperature of the wood, measuring the dry and wet bulb temperatures of the circulating heated air within the kiln, maintaining the wet bulb temperature of the circulating heated air substantially at a predetermined value, and in response to the measured surface temperature of the wood regulating the dry bulb temperature of of the circulating heated air and regulating the input of electrical energy for said dielectric heating so as to maintain a difference between the surface temperature of the wood and the wet bulb temperature substantially equal to a predetermined difference between the dry bulb temperature value and the wet bulb temperature value called for in a conventional schedule appropriate for the wood species, thickness and stage of drying, and said dielectric heating being selected from the group radio frequency heating and microwave heating.

2. A method according to claim 1, wherein the surface temperature of the wood is measured by an infrared radiation thermometer, said dielectric heating is radio frequency heating, the wood is arranged as layers of rows of timbers, the layers are spaced apart by electrode assemblies which pass radio frequency energy through the wood for said radio frequency heating, each electrode assembly comprises a perforated metal sheet with spacing strips on each side spacing the timbers from the metal sheet, and circulating air passes through the perforations in the metal sheet.

3. A method according to claim 2, wherein the perforated metal sheets are of aluminum, and the spacing strips are of a material selected from the group polystyrene and wood.

4. A method according to claim 2, wherein the radio frequency energy is fed to a central electrode assembly at a central position in the height of the layers of rows of timbers, and the layers of rows of timbers are heated by one portion of the radio frequency energy passing upwardly from the central electrode assembly through layers of rows of timbers and electrode assemblies to the uppermost electrode assembly, and another portion of the radio frequency energy passing downwardly through layers of rows of timbers and electrode assemblies to the lowermost electrode assembly. 

2. A method according to claim 1, wherein the surface temperature of the wood is measured by an infrared radiation thermometer, said dielectric heating is radio frequency heating, the wood is arranged aS layers of rows of timbers, the layers are spaced apart by electrode assemblies which pass radio frequency energy through the wood for said radio frequency heating, each electrode assembly comprises a perforated metal sheet with spacing strips on each side spacing the timbers from the metal sheet, and circulating air passes through the perforations in the metal sheet.
 3. A method according to claim 2, wherein the perforated metal sheets are of aluminum, and the spacing strips are of a material selected from the group polystyrene and wood.
 4. A method according to claim 2, wherein the radio frequency energy is fed to a central electrode assembly at a central position in the height of the layers of rows of timbers, and the layers of rows of timbers are heated by one portion of the radio frequency energy passing upwardly from the central electrode assembly through layers of rows of timbers and electrode assemblies to the uppermost electrode assembly, and another portion of the radio frequency energy passing downwardly through layers of rows of timbers and electrode assemblies to the lowermost electrode assembly. 